[3.4-ARCH-5] Redis as First-Class Development and Production Cache Backend

[Mawiguk0]

• Install the redis PHP extension in the production Dockerfile via install-php-extensions redis
• Add a cache Redis service to docker-compose-dev.yml with the eventula_manager_redis container name
• Add the app service's depends_on for eventula_manager_database, mailpit, and cache
• Document Redis connection in .env.example

Motivation: ARCH-2 #1587 and ARCH-3 #1898 both use Cache::supportsTags() to enable tag-based bulk invalidation. The array driver used previously does not support tags, so in production tags were silently skipped. Redis unlocks tag-based invalidation in all environments.